This invention relates generally to laminate films and sheets and more particularly to laminate films and sheets each having a layer of a polyethylene resin having simultaneously heat resistance and heat sealability.
The practice of laminating (uncrosslinked) polyethylene resins onto papers, cellophane, aluminum foil, synthetic resin films and sheets, and the like in order to impart heat sealability thereto, particularly in the food-packaging field, has been known in the prior art. However, the heat resistance of an (uncrosslinked) polyethylene resin is deficient with respect to conditions of ever-increasing severity in the food packaging field, for example, such as the recent trend toward the elevation of sterilization temperatures and the increase in speeds of automatic filling of packages with food. Consequently, an (uncrosslinked) polyethylene resin cannot be made to meet the recent requirements by techniques known in the present art.
For example, a packaging pouch or bag can be made from laminated sheets each having an (uncrosslinked) polyethylene layer by bringing the sheets with their polyethylene layers constituting their inner surfaces and heat sealing these surfaces together to form the bag. When this bag is filled with an article to be packaged and, in that state, is subjected to a sterilization process by boiling, or when the bag is used in an automatic filling and packaging operation wherein the article to be packaged is placed in the bag immediately after heat sealing, the heat-sealed parts tend to delaminate or be peeled away from each other, or the edges tend to be torn.
On one hand, the measure of crosslinking an (uncrosslinked) polyethylene resin thereby to improve its properties such as heat resistance is a common practice and is effective in the production of various formed articles. In the crosslinking methods practiced at present, however, while properties such as heat resistance can be improved, it is impossible to avoid a lowering of the heat sealability. For example, Japanese Pat. Publn. No. 1711/1972 discloses a method wherein crosslinkable polyethylene obtained by grafting an ethylenically unsaturated silane compound onto polyethylene is crosslinked by causing it to contact moisture in the presence of a silanol condensation catalyst. This method has attracted attention in recent years as a crosslinking method since it does not require special and expensive crosslinking equipment as in the conventional method depending on a peroxide and that depending on radiation. However, in this method, also, similarly as in the conventional methods, the crosslinked product does not exhibit heat sealability. Furthermore, the processing stability of this substance at the time of lamination with another material is poor, and this substance is also not satisfactory with respect to its odor.